1. Field of the Invention
The present invention relates to a method for producing monohydric alcohols from a monocarboxylic acid and, more particularly, to a method for producing a monohydric alcohol from a monocarboxylic acid using a hydrogenation catalyst which can directly hydrogenate the monocarboxylic acid in the gas phase regardless of whether or not the monocarboxylic acid contains water.
2. Description of the Related Art
To cope with the shortage of petroleum resources and solve global environmental problems, many researches have been conducted to develop an alternative energy source to petroleum used as an automobile fuel, and biofuels such as bioethanol and biodiesel are currently used as a petroleum-alternative transportation fuel in some countries. Meanwhile, biobutanol has advantages in that it causes no corrosion of automobiles and has a high boiling point, as compared to bioethanol. Further, biobutanol has advantages in that it can be directly used in an automobile system originally designed for using petroleum fuel without special modification of the automobile system, and it has higher fuel efficiency per volume than bioethanol. However, biobutanol preparation processes have a lower yield and productivity than bioethanol preparation processes, and a bio-process capable of economically preparing biobutanol has not yet been developed.
Accordingly, a new technology to utilize n-butyric acid from biomass as an intermediate of producing butanol is being considered as an alternative of the biobutanol processes. It is based on the bio-chemical composite technology, in which n-butanol is synthesized by preparing n-butyric acid from biomass through a bioprocess like fermentation and then reducing the n-butyric acid through a thermocatalytic process.
In petrochemical industry, n-butanol has been so far prepared in a large quantity through a hydroformylation reaction of propylene followed by a hydrogenation reaction. In chemical industry, n-butanol is widely utilized as a chemical solvent, a plasticizer, a reactant for butylamine, and the like.
It is a chemically easy reaction to prepare monohydric alcohols such as n-butanol by a reduction reaction of monocarboxylic acids such as n-butyric acid. However, an expensive strong reducing agent such as lithium aluminum hydride (LiAlH4) is used in the above-described chemical reaction, and thus the reduction reaction using such an expensive reducing agent is not suitable for mass production of commercially important monohydric alcohols such as n-butanol.
Meanwhile, in order to produce monohydric alcohols at the industrial scale, it has been used a hydrogenation reaction using hydrogen as a reducing agent on a hydrogenation catalyst. However, such a hydrogenation reaction cannot be generally applied to the direct hydrogenation of monocarboxylic acids. This is because conventional hydrogenation catalysts are often dissolved in a carboxylic acid which is a reactant and thus its catalytic activity is not maintained for a long period of time in the presence of the carboxylic acid, or because components of hydrogenation catalysts cause the decarboxylation of the carboxylic acid to lower selectivity in the direct hydrogenation reaction of the carboxylic acid.
Therefore, most hydrogenation processes of monocarboxylic acids have been performed by a method involving two steps of esterifying carboxylic acid with methanol or ethanol to obtain an esterified compound and then hydrogenating the obtained esterified compound to prepare a monohydric alcohol. For example, 1,4-butanediol is prepared by hydrogenating an esterified compound obtained from maleic acid or maleic anhydride with methanol or ethanol [U.S. Pat. Nos. 6,100,410, 6,077,964, 5,981,769, 5,414,159, and 5,334,779].
However, since these processes additionally includes the esterification reaction process and processes of recovering and purifying alcohols used in the esterification reaction in the hydrogenation of carboxylic acid, and processes for recovering and purifying unreacted esterified compounds remaining after the hydrogenation reaction, the reaction processes become complicated and production costs are disadvantageous.
In order to solve the above-mentioned problems, many researches have been conducted to simplify reaction processes for producing monohydric alcohols.
For example, U.S. Pat. No. 6,495,730 and cited references therein disclose a hydrogenating catalyst system for preparing 1,4-butanediol by directly hydrogenating maleic acid or succinic acid under a reaction condition in which an excessive amount of water is supplied as compared with the amount of carboxylic acid [wherein, the hydrogenating catalyst system includes ruthenium-tin/activated carbon; ruthenium-iron oxides; ruthenium-tin/titania or alumina; ruthenium-tin and a component selected from alkali metals and alkaline earth metals; a component selected from tin-ruthenium, platinum and rhodium; and, ruthenium-tin-platinum/activated carbon].
Further, U.S. Pat. No. 4,443,639 discloses a ruthenium-based catalyst, ARuDEOx (A=Zn, Cd, Ni and mixtures thereof, and E=Fe, Cu, Rh, Pd, Os, Ir, Pt and mixtures thereof) as a hydrogenation catalyst of n-butyric acid, wherein it is exemplified that n-butanol is obtained in the presence of water and n-butylbutyrate is obtained in the absence of water.
However, these conventional technologies are problematic in that, since an excessive amount of water is used to prepare monohydric alcohols from carboxylic acids (for example, U.S. Pat. No. 4,443,639: 10 wt % of aqueous acid is used), an amount of generated waste water and energy consumption are high and productivity is low (for example, LHSV: 0.1 hr−1 or less), and a high pressure of 60 atms or more is required.
For this reason, in order to produce monohydric alcohols at the industrial scale through the hydrogenation of monocarboxylic acids, there have been demands for developing an economical preparation technology.
Hence, the present inventors have conducted many experiments to solve the above-described problems of the conventional technologies using ruthenium-based catalysts to hydrogenate carboxylic acids, in which the problems are that a high reaction pressure is required; an excessive amount of water should be simultaneously supplied to reactants; industrial application is difficult due to low productivity, and the like. As a result, they have found that activity and selectivity of carboxylic acid hydrogenation reaction are remarkably improved in case of using a catalyst which contains a proper concentration of ruthenium (Ru) and is modified with tin (Sn) using zinc oxide (ZnO) as both a catalyst support and an active promoter, and that monohydric alcohols can be efficiently produced by hydrogenating monocarboxylic acids in the gas phase under the presence of the catalyst using a fixed-bed reaction. Based on these findings, the present invention has been completed.